The present invention generally relates to an image capture device, such as a digital camera, and more specifically, to an image capture device having an autofocus assembly operative to diminish a backlash condition.
Conventionally, the image capture device, such as a still camera and a video camera employs an autofocus assembly to automatically adjust a focus condition of an optical image of a subject. The autofocus assembly arranged in a traditional image capture device has a range-finding unit that is configured to measure a distance between the subject and the device.
Additionally, the conventional device includes a focus driving unit that is arranged to drive a focusing lens group to the focusing position corresponding to the distance measured by the range-finding unit. This operation is called an external-metering AF (auto focus) operation. The external-metering AF employs two known methods of operation corresponding to different range-finding techniques. One is called a passive AF operation.
Currently, digital cameras are operative to project the optical image of the subject on the image pickup unit, such as a Charge Coupled Device CCD or a Complementary Metal Oxide Semiconductor-type image sensor, and convert the projected image into an electronic signal. Such devices obtain the image signals successively while controlling driving of the focusing lens group which is part of a shooting optical system; evaluate the contrast of the image projected onto the image pickup unit based on the image signal obtained, in real time; and control the focusing lens group to stop at a position that is evaluated as optimal. This auto focus operation is referred to as a CCD-AF operation or a contrast AF operation.
Though the CCD-AF operation provides high focusing precision, the CCD-AF operation also has two problems. The first problem is that necessary time to determine processing of the CCD-AF operation (i.e., necessary time to complete the AF operation) is relatively long. Likewise, the second problem is that a user cannot capture images before the completion of the AF operation, when the user wants to capture an image having objects that move at high speed. The CCD-AF operation may be not able to execute focusing movement adequately so that the image capture device cannot evaluate remarkable contrast of the image projected onto the image pickup unit, for example, in a dark condition having a decreased level of overall contrast. Conversely, the external-metering AF functions to focus on the subject comparatively well so that the necessary time (time-lag) for the AF operation is shorter than the CCD-AF operation. CCD-AF is not readily influenced by contrast of the subject; and the user can capture images the subject at desired timing.
A hybrid AF operation is proposed in Japanese Patent Publication of unexamined Application No. 2001-255456 gazette. This document describes a configuration to execute the CCD-AF operation at only a peripheral range according to the distance of the subject as detected by the range-finding unit. The hybrid AF operation arranged in the still camera is employed together with the external-metering AF operation using same general range-finding information measured by the range-finding unit. In this way, the user can capture images according to shooting conditions by switching the operation between the external-metering AF operation and the CCD-AF operation (including the hybrid AF operation) based on condition of the subject.
Backlash is present in order to smooth an interlocked driving between a driving member and a driven member at a coupling-part between the shooting optical system and a driving system for the shooting optical system. Specifically, backlash is a mechanical “slack” between movable elements of the optical system and movable elements of the driving system of the shooting optical system. Thus, backlash must be compensated to precisely control a position of the shooting optical system.
In other words, the shooting optical system of the digital camera as the image capture device is typically driven by a pulse motor, for instance, and a push-out (i.e., extended) position of the focusing lens group is configured according to a pulse-number applied to the pulse motor. Therefore, a stopping-position of the focusing lens group may have an accidental error in accordance with a driving direction (a push-out direction or a pull-back direction) from a current position of the focusing lens group toward the focusing position by hysteresis with backlash. This backlash occurs even if the predetermined pulse-number to drive the focusing lens group to the predetermined push-out position is applied to the pulse motor.
A method of addressing backlash is proposed on Japanese Patent Publication of unexamined Application No. 7-063973 gazette. A backlash removing operation is proposed to control the driving direction of the focusing lens group among predetermined range before disengaging the drive in the push-out direction. In other words, when the focusing lens is driven on the push-out direction that is not affected by the backlash, the focusing lens is stopped at driving-target position on the push-out direction. When focusing lens is driven on the pull-back direction that is affected by the backlash, the focusing lens is driven on the pull-back direction to a position beyond the driving-target position, the focusing lens group is turned over on the push-out direction to the driving-target position after reaching to the position; and is stopped. In this manner, the camera can cancel the backlash substantially by above-mentioned one-way-driving.
However, in a general case of the driving system of the focusing lens group having amount of the backlash defined as K; amount of outreached driving defined as Alpha; amount of the backlash-removing operation L defined as K+Alpha; the pulse-number corresponding to amount of the backlash-removing operation L defined as 10 pulses; and a pulse rate of the pulse motor defined as 700 pps, a pulse number for extra driving requires 20 pulses (i.e., 10 pulses*2 (both ways)) and operation time TB for extra driving requires almost 30 ms (i.e. 20 pulse/700 pps=28.6 ms). Also, when the operation time TT to turn over direction for the pulse motor is defined as almost 10 ms, total necessary time T (=TB+TT) for the backlash-removing operation becomes almost 40 ms.
Yet, when the above-mentioned backlash-removing operation is executed during the CCD-AF operation (including the hybrid AF operation), the increased time necessary to complete the CCD-AF operation is marginal so that necessary time for the CCD-AF operation is about 500-1000 ms. Thus, the CCD-AF operation takes more time in comparison with the external-metering AF operation. On the other hand, however, the user may feel that the focusing movement is sluggish so that the increased time necessary to complete external-metering AF operation is larger than the CCD-AF operation according to the shooting-condition, when the above-mentioned backlash-removing operation is executed during the external-metering AF operation